Photoelectric imaging devices are well-known in the art and produce machine-readable data which is representative of an image of an object, e.g. a page of printed text. Examples of such photoelectric imaging devices include telefax machines, photocopy machines and optical scanning devices.
Many photoelectric imaging devices employ line-focus systems which image an object by sequentially focusing narrow "scan line" portions of the object onto a linear photosensor array by causing relative movement between a scanning head and the object being scanned.
In a line-focus system, a light beam from an illuminated line object is imaged by a lens onto a linear photosensor array which is positioned remotely from the line object. The linear photosensor array is a single dimension array of photoelements which correspond to small area locations on the line object. These small area locations on the line object are commonly referred as "picture elements" or "pixels." In response to light from its corresponding pixel location on the line object, each photosensor pixel element in the linear photosensor array (sometimes referred to simply as a "pixel") produces a data signal which is representative of the light intensity that it experiences during an immediately preceding interval of time known as a sampling interval. All of the photoelement data signals are received and processed by an appropriate data processing system.
In a color line focus system, a number of photosensor arrays may be used to acquire a corresponding number of distinct color components. Each photosensor array may be used to acquire a separate color component (typically red, green and blue components). Many color line focus systems use a plurality of photosensors, each of which has a different color filter associated therewith. In this manner, each photosensor is able to acquire color data corresponding to a single color component (e.g., red, green and blue).
Other color line focus systems employ beam splitter devices for spectrally separating an imaging light beam into color component beams. These separate color component beams are projected onto separate linear photosensor arrays. Still other color line focus systems project color component images onto a single linear array in a series of separate scanning passes.
The construction and operation of color line focus systems employing beam splitter assemblies and photosensor arrays are disclosed in the following U.S. Pat. No. 5,410,347 of Steinle et al. for COLOR OPTICAL SCANNER WITH IMAGE REGISTRATION HOLDING ASSEMBLY; U.S. Pat. No. 4,870,268 of Vincent et al. for COLOR COMBINER AND SEPARATOR AND IMPLEMENTATIONS; U.S. Pat. No. 4,926,041 of Boyd for OPTICAL SCANNER (and corresponding EPO patent application no. 90306876.5 filed Jun. 22, 1990); U.S. Pat. No. 5,019,703 of Boyd et al. for OPTICAL SCANNER WITH MIRROR MOUNTED OCCLUDING APERTURE OR FILTER (and corresponding EPO patent application no. 90312893.2 filed Nov. 27, 1990); U.S. Pat. No. 5,032,004 of Steinle for BEAM SPLITTER APPARATUS WITH ADJUSTABLE IMAGE FOCUS AND REGISTRATION (and corresponding EPO patent application no. 91304185.1 filed May 9, 1991); U.S. Pat. No. 5,044,727 of Steinle for BEAM SPLITTER/COMBINER APPARATUS (and corresponding EPO patent application no. 91303860.3 filed Apr. 29 1991); U.S. Pat. No. 5,040,872 of Steinle for BEAM SPLITTER/COMBINER WITH PATH LENGTH COMPENSATOR (and corresponding EPO patent application no. 90124279.2 filed Dec. 14, 1990 which has been abandoned); U.S. Pat. No. 5,227,620 of Elder, Jr. et al. for APPARATUS FOR ASSEMBLING COMPONENTS OF COLOR OPTICAL SCANNERS (and corresponding EPO patent application no. 91304403.8 filed May 16, 1991) and U.S. Pat. No. 5,646,394 of Steinle et al. for IMAGING DEVICE WITH BEAM STEERING CAPABILITY, which are all hereby specifically incorporated by reference for all that is disclosed therein.
A hand-held line focus system is a photoelectric imaging device which is moved across a scanned object, e.g. a page of text, by hand. Optical systems for hand-held line focus systems must generally be very compact due to the relatively small size of hand-held scanning devices.
The construction and operation of hand-held line focus systems are disclosed in the following U.S. Pat. No. 5,381,020 of Kochis et al. for HAND-HELD OPTICAL SCANNER WITH ONBOARD BATTERY RECHARGING ASSEMBLY and U.S. Pat. No. 5,306,908 of McConica et al. for MANUALLY OPERATED HAND-HELD OPTICAL SCANNER WITH TACTILE SPEED CONTROL ASSEMBLY (and corresponding EPO patent application no. 94301507.3 filed Mar. 2, 1994) and in the following U.S. patent application Ser. No. 08/601,276 of Ronald K. Kerschner et al., filed Jan. 29, 1996, for HAND-HELD SCANNING DEVICE and Ser. No. 08/592,904 of Ronald K. Kerschner et al. filed Jan. 29, 1996, for SCANNING DEVICE WITH NON-CONTACT OPTICAL COMPONENTS, which are all hereby specifically incorporated by reference for all that is disclosed therein.
In a line focus system, optical components, including a lens as previously mentioned, are generally arranged between the object to be imaged and the photoelectric sensing device, e.g., a linear photosensor array. The optical components serve to direct and focus the light beam from the scan line area of the object being imaged onto the linear photosensor array.
Typically, these optical components are mounted, along with the linear photosensor array, within a housing which is moveable relative to the object being scanned. The housing, in turn, generally includes an elongated opening or slot to enable the light beam to enter the housing and impinge upon the optical components housed therewithin. In order to permit unobstructed passage of the light beam into the housing, the width of the slot must be at least as wide as the light beam at the point where it enters the housing. The width of the slot, however, must generally be formed larger still in order to accommodate any drift in the scan line which may occur during operation.
The components, e.g., the mirrors, lens and photosensor of a typical photoelectric imaging apparatus are generally attached to the housing which may, for example, be formed of a plastic material. This plastic material often has a relatively high coefficient of thermal expansion, that is, an increase in temperature causes the material to expand a relatively large amount and a decrease in temperature causes the material to contract a relatively large amount. As can be appreciated, this expansion and contraction causes relative movement between the various components, e.g., the mirrors, lens and photosensor, housed within the reciprocal housing. This relative movement, in turn, causes the light beam and, thus, the scan line to drift. As previously pointed out, the housing slot must be large enough to accommodate this scan line drift since, if the scan line drifts beyond the edge of the slot, the optical components will no longer be able to image any portion of the object onto the linear photosensor array.
A typical photoelectric imaging apparatus is designed to operate over a range of temperatures. Accordingly, the slot of a typical photoelectric imaging apparatus must be made sufficiently wide to accommodate the scan line drift encountered over this operating range of temperatures. Providing a wide slot, however, is disadvantageous for several reasons. One reason is that a wide slot allows stray light to enter the housing. This stray light, in turn, causes various optical problems such as increased sensitivity to contamination and a limited dynamic range. The use of a wide slot also limits the types of color separation methods which may be used within the housing. A trichromatic beam splitter arrangement, for example, is not practically useable in conjunction with a large slot.
Accordingly, it would be desirable to provide a photoelectric imaging device which overcomes the problems described above associated with temperature induced scan line drift.